This invention relates to composite blades for use in gas turbine engines and, more particularly, to improved leading edge foreign object damage protection for use therein.
For many years attempts have been made to replace the relatively heavy, homogeneous metal blades and vanes of gas turbine engine compressors with lighter composite materials. The primary effort in this direction has been toward the use of high strength, elongated filaments composited in a lightweight matrix. Early work involved glass fibers and more recent efforts have been directed toward the utilization of boron, graphite and other synthetic filaments. These later materials have extremely high strength characteristics as well as high moduli of elasticity which contribute to the necessary stiffness of the compressor blades and vanes.
Many problems have confronted the efforts to utilize these filaments, particularly in adapting their unidirectional strength characteristics to a multidirectional stress field. To a large extent, these problems have been overcome and composite blades have been demonstrated with performance characteristics, in many areas, equal to or better than their homogeneous metal counterparts in addition to providing the expected and significant weight reductions.
However, composite blades have not yet been introduced into operational service due, to a large extent, to their vulnerability to what is referred to as foreign object damage (FOD). Many types of foreign objects may be entrained in the inlet of an aircraft gas turbine engine ranging from large birds, such as seagulls, to hailstones, sand and rain. Damage from foreign objects takes two forms. Smaller objects can erode the blade material and degrade the performance of the compressor. Impact by larger objects may rupture or pierce the blades. Portions of an impacted blade can be torn loose and cause extensive secondary damage to downstream blades and other engine components.
In this regard, the consequences of foreign object damage are greatest in the low pressure compressors, or fans, of high bypass gas turbine engines. However, these components offer the greatest potential in weight reduction due to their large tip diameters, as great as eight feet, and spans in the order of three or more feet.
The vulnerability of composite blades to foreign object damage is due to two factors. First, the lightweight matrix material employed, generally polymeric resins or metals such as aluminum, is relatively soft. Second, the high strength filaments are relatively hard and brittle.
From this it would seem evident that a protection system involving a hard surface coating should be provided for these composite blades and vanes. This is even more evident from the early recognition in the aviation industry that such protective systems were desirable for wooden propeller blades and propeller blades formed of early composite materials such as cloth fabric in a phenolic resin matrix. Many such systems have been proposed. They include claddings of various compositions applied to the leading edge portion or the entire surface of a propeller.
One particularly promising system for protecting the leading edge of composite blades and vanes is disclosed and claimed in U.S. Pat. No. 3,762,835, "Foreign Object Damage Protection for Compressor Blades and Other Structures and Related Methods," Carlson et al, which is assigned to the same assignee as the present invention and the disclosure of which is incorporated herein be reference. Therein, a filament composite compressor blade is provided with a fine wire mesh subsurface layer wrapped around its leading edge. This subsurface layer is clad with nickel and provides a protection against impact damage by large and small foreign objects such as stones and sand. Where the subsurface mesh layer is bonded in place by a non-conductive adhesive, the adhesive in the interstices of the mesh is coated with silver and the nickel is atomically bonded to the nubs of the mesh forming a metallic strip.
However, it has been found that in some instances where the leading edge FOD protection comprises such a metallic strip wrapped about the fan blade leading edge, this protection may be lost during impact by medium-sized birds due to partial delamination of the blades. This, in turn, can result in secondary engine damage as the FOD protection strip is ingested through the engine. Additionally, engine imbalance at high speeds can cause further damage.
Observation of high speed films taken of a rotating laminated composite fan blade during such an impact reveals that, sequentially, the FOD protection strip is broken at the location of impact; severe bending of the leading edge normal to the blade chord occurs; and, the entire leading edge FOD strip is lost after impact is completed due to delamination within the first few filament laminars (or plies) beneath the FOD strip. The outermost filament laminars separate from the remainder of the blade carrying with them the leading edge FOD strip since these filament laminars, by themselves, cannot carry the (now delaminated) leading edge and fracture near the root of the blade. Hence, the entire leading edge FOD strip is lost.